Write the following sets in the set-builder form $A=\{3,6,9,12\}$

$\begin{array}{1 1}(A)\;A=\{\text{x : x is a natural number multiple of 3 and x < 15}\}\\(B)\;A=\{\text{x : x is a natural number multiple of 4 and x < 16}\}\\(C)\;A=\{\text{x : x is a natural number multiple of 5 and x < 15}\}\\(D)\;A=\{\text{x : x is a natural number multiple of 6 and x < 36}\}\end{array} $